I have not seen a magic band up close yet. Is there anything on it that someone can see in a picture that would allow thieves to access an account?
Very little to worry about with a picture. There is an ID code on the inside of the band. However even if someone did get a photo of the ID number they would still need access to internal Disney systems and databases to get any sort of personal info. From the outside they are fairly plain.
However your credit card and/or meal plan can only be charged using a PIN and your ticket to the park can only be used with your fingerprint.
That said, i'd be more concerted with someone using a long range RF scanner to get your ID than with someone using the number on the back of the band to clone it.